1. Field of the Invention
The present invention relates to a complementary metal oxide semiconductor (CMOS) image sensor and a method for fabricating the same.
2. Description of the Related Art
An image sensor is a semiconductor device that converts an optical image into electric signals. Examples of the image sensor include a charge coupled device (CCD) and a CMOS image sensor.
In the CCD, metal oxide semiconductor (MOS) capacitors are very closely adjacent to each other, and charge carriers are stored in and transferred into and/or from the MOS capacitor.
Through a CMOS technology using a control circuit and a signal processing circuit as peripheral circuitry, the CMOS image sensor includes a particular number of MOS transistors per unit pixel, and employs a switching method that sequentially detects outputs of the unit pixels using the MOS transistors.
However, a method for driving the CCD is relatively complex and consumes relatively large amounts of power. Moreover, there are more disadvantages in that processes are more complex due to the large number of mask process steps, and it is hard to integrate a signal processing circuit into a CCD chip. To overcome these problems, the CMOS image sensor has been under development.
The CMOS image sensor includes a photodiode and one or more MOS transistors in a unit pixel, and sequentially detects signals using a switching method to display an image. In a method for fabricating the CMOS image sensor, power consumption and the number of masks are less than those of the CCD image sensor. The number of masks for fabricating the CMOS image sensor is 20, and 20 to 40 for fabricating the CCD image sensor. Additionally, the CMOS image sensor can be placed on one chip with a plurality of signal processing circuits, and also can gain attentions for a next generation image sensor. The CMOS image sensor is widely used for an application device such as a digital still camera (DSC), a PC camera, and a mobile camera.
However, in related art CMOS image sensors, the transmittance of light incident to a photodiode through a microlens may deteriorate. Moreover, since a focal length of a microlens becomes longer when light is incident to a photodiode through the microlens, the sensitivity of the related art CMOS image sensor may deteriorate.